In the process of making paper which begins with an aqueous suspension of fibers deposited onto a traveling paper-forming fabric, the formation of the paper occurs in different steps during which the paper being formed is at least partially supported on a traveling belt having a fabric designed to accomplish processing of the paper at a particular step in the process. In the dryer section of the paper making machine, heated cylinders are arranged so that an endless traveling dryer fabric holds a paper web in contact with the drying cyliners for drying of the paper web.
Woven dryer fabrics have been heretofore provided which consist of synthetic yarns woven together in a relatively open weave which is fluid permeable. The weave pattern of the fabric is carried out to satisfy the permeability and other requirements which are determined by the grade of paper being made and the operating conditions. The heated cylinders of the dryer section of the paper making machine are typically heated by introducing steam into the interior of the cylinders, by infrared radiation, or by other suitable means. It has been found that the temperature of the heated cylinder varies from one end to the other by a considerable amount. This creates the possibility that some portions of the paper web will be subjected to greater drying action than other portions of the paper web which creates a non-uniform moisture profile in the paper web across the width of the web. It has also been found that water vapor accumulates in the central portion of the paper web due to the fact that the lateral edges of the paper web lie closer to the surrounding atmosphere and may be more readily ventilated.
To eliminate a higher moisture content near the center of the paper web than at its edges, dryer fabric having less permeability at the edges of the fabric has been provided. In this manner, it has been attempted to provide a uniform moisture profile of the paper web across its width from one end of the heating cylinder to the other. In U.S. Pat. No. 3,867,766, it has been proposed to vary the number of warp elements at the lateral edges of the dryer fabric to produce a reduced permeability at the edges. Alternate methods of reducing the permeability of the lateral edges of the dryer fabric have included varying the diameter or the density of the warp yarn elements at the lateral edges of the fabric. In U.S. Pat. No. 4,460,023 it is proposed to insert an additional pick of the weft yarn across the width of the fabric at the lateral edges of the fabric to reduce the permeability at the edges. The remainder of the pick across the width of the fabric is severed so that the pick of the weft yarn only exists at the lateral edges.
Another method of reducing the permeability of paper making fabric at the edges of the fabric is disclosed in U.S. Pat. No. 4,426,795 wherein treatment of the edges is carried out to effect increased drying efficiency at the center of the belt to compensate for decreased drying efficiency at the center of the paper web being dried.
In the above proposals where reduction of permeability at the lateral edges of the fabric is achieved by weaving, the result has been that the tension of the fabric across its width is made to vary. This is because the additional warp elements or warp elements of different characteristics are under different tensions in the weave. The resulting woven fabric has a variable tension profile across its width wherein the flexibility of the fabric is decreased at the lateral edges. In repeated traveling about the belt rollers on the paper-making machine, the non-uniform tensions and flexibilities of the fabric can cause distortion of the fabric. The center of the fabric tends to become bowed and the open spaces of the fabric in the medial portion of the fabric begin to close in whereby the permeability profile of the fabric is lost. Moreover, the less flexible lateral edges of the fabric are susceptible to flex fatigue during repeated travel of the fabric in an endless configuration about the belt rollers.
Accordingly, an important object of the present invention is to provide a paper-making fabric for a paper making machine wherein the permeability profile of the fabric is varied across its width, yet uniform tension is provided across the width of the fabric.
Still another important object of the present machine is to provide a dryer fabric for a dryer section of a paper-making machine having reduced permeability at its lateral edges and uniform tension across its width.
Still another important object of the present invention is to provide a woven dryer fabric for a dryer section of a paper making machine wherein additional closure elements are provided in the warp direction of the fabric which are not woven in the fabric so that the presence of the closure elements does not effect the tension of the woven elements in the fabric.
Still another important object of the present invention is to provide a method of weaving a dryer fabric for a dryer section of a paper making machine wherein a desired permeability profile may be provided across the width of the fabric, yet at the same time, the fabric may be provided with a uniform tension profile across the width of the fabric in a woven structure.